Context 161 - September 2019

6 C O N T E X T 1 6 1 : S E P T E M B E R 2 0 1 9 Periodically APT Bulletin It is always a pleasure to open a review of periodicals with APT Bulletin for 2018 (Vol XLIX, No 4) as a benchmark for heritage standards and professional best practice that continues to inform the proper management of the sector and move our knowledge base forward. The latest issue commences with an evaluation by Alice Louise Gilmore and Frank Matero of the performance testing of acid stains on historic concrete. The authors note that the versatility of concrete owes much to its surface effects but that hitherto there has been little contemporary discussion of the preservation of its surfaces. This has presented considerable challenges to a number of time- honoured conservation issues, including the expected durability and acceptable aging of a building’s fabric. The paper usefully reviews the chemistry of acid staining of concrete for its aesthetic effect; the documentation of such intentional historic surface stains; and the replication of original external finishes by facsimile reproduction using accelerated weathering techniques, following experimentation on a nationally important mass concrete 1955 building at the Grand Teton National Park,Wyoming. Thomas Leslie looks at the development of insulated glazing as a modern historic material over the period 1930 to 1980. He considers its evolution that promised building occupiers the opportunity to sit comfortably in their rooms looking through huge, vaporously thin picture windows irrespective of the outside temperature. Although early air-conditioning was technically and economically successful, the thermal inefficiencies of glass made such systems ineffective and expensive to operate. The paper is helpful in summarising the early experiments with insulating glass and the troubled development of effective sealed units, while rival products in the form of glass blocks became a signature feature of streamlined modern-style buildings of the 1930s. The 1940s saw the rapid adoption of Thermopane as doubled glazing was perfected and unit costs of production fell to the point where its use became economically feasible, and even preferable to other cladding materials, such as brick and stone. While insulated glazing itself is a nearly invisible technology and the economics of windows have always kept installations as thin as possible, it has nevertheless been critical in the development of curtain walls in commercial construction and residential picture windows. This has implications for ongoing heritage management. The glass curtain wall would be thermally impossible without the insulating qualities of Thermopane and its successors, as would the expanses of glass in the large windows, walls and sliding doors that defined mid-century residential modernism, now being evaluated as heritage assets Coupling the desire for clear vision and natural daylight with the provision of insulation enabled the environmentally impractical fantasy of the early modernists – the glass skin – to become common in the later 20th century. Although not a universal solution, insulated glazing has become a pervasive and reliable technology in modern construction, taken for granted by architects. The history of insulated glass shows that advances by industry in the middle of the 20th century provided a material as vital to technically proficient construction as the more celebrated examples of steel, aluminum or concrete that now fall to be considered for heritage protection. Another paper of interest in the bulletin concerns the restoration, after years of assessment and planning, of the spectacular mosaic ceilings of the 1927 Beaux-Arts Wellington Building in Ottawa. The paper includes an informative description of the initial structural monitoring process, the scope of the intended works and the goals of the proposed treatment, and a detailed summary of the completed repairs.